1. Field of Invention
Embodiments of the present invention relate to a method of applying a spot varnish to selected areas of a xerographic image, thereby achieving a xerographic image having different image regions with different gloss finish appearance. Further embodiments of the invention relate to emulsion aggregation toners having gloss tunability that are used as the spot varnish material.
2. Description of Related Art
In printing full color images by offset press, the gloss of the image is usually determined by the gloss of the paper being printed. In other words, to make a glossy image on an offset press, one would use a glossy (usually coated) paper, while to make a matte image, one would use a matte (uncoated or matte coated) paper. This is because in an offset press, a thin layer of ink is laid down which follows the surface roughness of the substrate. In some cases however, one would like to have certain areas of a single page appear glossy, while other areas appear matte. This can produce aesthetically striking results because glossy color images tend to have much more vibrant colors than matte color images. In an offset press, this selective gloss can be achieved by the application of spot varnish. This is a transparent ink coating that is applied over selected areas of a full color offset print to impart gloss to those areas.
Compared to offset presses, xerographic engines deposit a relatively thick toner layer on the paper, and thus to some degree the gloss of a color xerographic image can be controlled independently of the gloss of the substrate. The gloss of a xerographic image is determined largely by the toner mass per unit area (TMA) that is deposited on the substrate, by the melt rheological properties of the toner, and also by the nature and texture of the fuser roll that contacts the image during the fusing step. For example, XEROX highlight color machines such as 4850 and 4890 produce matte color images, while XEROX pictorial color machines produce glossy full color images even on matte uncoated paper. Neither type of machine, however, currently has the capability of producing a color page in which part of the page is glossy, and part is matte.
Moreover, newer high speed, full color xerographic machines are meant to compete with offset presses for short run color printing. If such machines are to effectively compete with offset presses, they must offer all the capabilities of an offset press, including the ability to provide spot varnish.
Accordingly, there is a need for a xerographic equivalent of a spot varnish application in offset printing.